To provide further information on the role of donor immune T cells transferred to CY-treated MCA/76-9 sarcoma-bearing mice and to assess whether the induction of tumor regression was mediated by donor or host T cells, or both, the following experiments were undertaken. Radiation chimeric mice (B6\\\B6D2F1 and DBA/2\\\B6D2F1 mice) were established. The blood and the lymphoid tissues of these mice expressed the H-2 haplotype of the reconstituting bone marrow; that is, there were no detectable F1 cells remaining in these tissues. About 12 weeks after reconstitution, the mice were injected with sarcoma cells followed 10 days later by CY and immune F1 hybrid cells. After a further 10 days, the cellular composition of the regressing tumor mass was analyzed using glucose phosphate isomerase (Gpi) as a qualitative marker for the presence of B6 or DBA isozymes (Gpi-1b and 1a, respectively) and monoclonal anti-H-2 antibody for the presence of cells bearing the H-2b or H-2d determinants. It was seen that following electrophoresis of tumor-associated cell extracts that all three bands of the B6, DBA, and the F1 hybrids were expressed. However, on fractionation of the tumor cells into macrophages, neutrophils, and T lymphocytes, it was demonstrated by both the Gpi assay and H-2 typing that macrophages and neutrophils were derived from the bone marrow used to reconstitute the lethally irradiated F1 hybrid mice originally; that is, either from the B6 or DBA marrow, while the T cells were F1 in origin. It is, therefore, likely that donor T-cell immune responses are amplified under these conditions at the tumor site in the absence of host T-cell responses. This is particularly interesting in light of the finding that the radiation chimeras expressed graft-versus-host-associated immune suppression, implying that the adoptively transferred immune cells circumvented the suppressor mechanisms. This was probably achieved by the action of CY on the suppressor mechanisms. (MB)